Generally, a water level measuring system has been used for measuring the water levels of rivers, lakes or streams. Information obtained by measuring of the water level measuring system, such as the current water levels, the change of the water levels, are used for many purposes such as bridge management, prediction of flood during a rainy season, or traffic control for security in case of a water level equal to or above a predetermined level. Water level meters used for a water level measuring system are classified into a contact type water level meter that measures a water level by contacting water, and a non-contact type water level meter that measures a water level by projecting ultrasound or laser beams to water.
Such a water level measuring system is usually installed at a bridge pier in a stream or a lake. Considering that the water level measuring system is used to predict flood or control traffic safely, when the water level of a river or a stream reaches or exceeds a predetermined level or there is a rapid change in the water level, these events should be notified to persons walking along a bridge or a manager as soon as possible.
However, it is very difficult to monitor a conventional water level measuring system at night, during raining, or in case of a rapid change in water level with the naked eye. Even if the water level can be observed, there is a high probability of error in the observed water level due to lack of visibility. Especially at the flood site, fluctuating water levels should be monitored with the naked eye. Therefore, the conventional water level measuring system is almost useless.